An anti-establishment candidate has been elected Rome's first female mayor after Sunday's runoff elections in Italy.

Thirty-seven-year-old lawyer Virginia Raggi led by a 2-1 margin over Premier Matteo Renzi's chosen candidate, Roberto Giachetti, who conceded defeat less than an hour after polls closed.

Raggi represents the populist Five Star Movement (M5S) founded just seven years ago by comedian Beppe Grillo.

Raggi had campaigned on ending corruption in the Italian capital and reversing the trend of severely failing public services across Rome.

"I will work to bring legality and transparency" to Rome's administration, Raggi told supporters early Monday. "The citizens of Rome have won," she declared before pledging that "with us a new era begins."

M5S had become the leading opposition party against the ruling coalition of Renzi and his center-left Democratic Party.

Rome was one of more than 100 cities and towns across Italy that held local runoff elections Sunday, representing more than a quarter of the country's population.